The conservative majority’s endorsement of nearly unchecked presidential power in this context is all the more disturbing given this administration’s policies at America’s southern border, which include separating children from their parents and prosecuting those trying to come here from brutally violent countries in Central America.

It’s no small paradox that the justices chose Tuesday’s ruling to formally overturn, at long last, one of the greatest abominations in the court’s history, Korematsu v. United States, the 1944 decision that upheld President Franklin Roosevelt’s order to lock up thousands of Japanese-Americans for years based on nothing but their ancestry — and based on a fabricated claim that our national security demanded it. The Korematsu ruling was “morally repugnant,” the court said, and was “gravely wrong the day it was decided.” That’s surely correct. It’s also easy to say from a distance of 74 years, protected by the warm embrace of history’s consensus. It’s much more important to say it in the moment — as Justice Robert Jackson did in his dissent from the Korematsu decision, which he warned was “a far more subtle blow to liberty than the promulgation of the order itself.”

White racial fear has always been at the core of Mr. Trump’s worldview. What’s so dangerous about Tuesday’s ruling is that the Supreme Court has now implicitly blessed his use of this strategy as a political organizing tool and as a governing philosophy.

On Jan. 27, 2017, as Mr. Trump signed the first version of the travel ban, he read out its official title, “Protecting the Nation From Foreign Terrorist Entry Into the United States,” then looked up and said, “We all know what that means.”